The present application is based on Japanese Patent Application No. 11-107865 filed Apr. 15, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an improvement of a quantum well type light-emitting diode (LED) having at least one light-generating layer of a quantum well structure and a light resonator which is constituted by a pair of reflecting layers between which the light-generating layer is interposed.
2. Discussion of the Related Art
There has been known a quantum well type light-emitting diode formed of a plurality of semiconductor layers which are laminated on a semiconductor substrate, wherein the plurality of semiconductor layers include at least one light-generating layer of a quantum well structure which generates a light and has a thickness value smaller than the wavelength (100 xc3x85, i.e., 10 nm) of the electron wave, and a light resonator consisting of a pair of reflecting layers which are located on the opposite sides of the light-generating layer, for reflecting the light generated by the light-generating layer. By applying an electric current between two electrodes respectively formed on opposite two major surfaces of a laminar structure consisting of the plurality of semiconductor layers, the light generated by the light-generating layer is emitted from one of the two opposite major surfaces of the laminar structure which is remote from the substrate. In the thus constructed quantum well type light-emitting diode, the electron wave in the light-generating layer and the optical wave in the light resonator are coupled together, so that the light-generating layer generates a light only in a resonance mode. This phenomenon is generally referred to as cavity QED effect. Owing to this cavity QED effect, the above-described light-emitting diode is capable of emitting a light which has a high degree of directivity and a narrow line width, so that the emitted light does not suffer from total reflection on the crystal face, assuring a high degree of external quantum efficiency. Examples of such a light-emitting diode are disclosed in JP-A-4-167484 and JP-A-10-27945.
The above-described quantum well type light-emitting diode having the light resonator in which there is disposed the light-generating layer of a quantum well structure is capable of assuring a high degree of external quantum efficiency. This light-emitting diode, however, undesirably tends,to cause continuously induced or stimulated emission in the light resonator when a relatively large magnitude of electric current is applied to the diode for improving its light emission output. Accordingly, the light-emitting diode constructed as described above is likely to cause laser oscillation for the following reasons.
Described more specifically, when the plurality of semiconductor layers are formed by crystal growth on the substrate, the lattice constants of the semiconductor layers are generally matched to one another. Accordingly, when the light-generating layer is formed to have a quantum well structure, a band splitting occurs in the valence band of the light-generating layer, such that the valence band of the light-generating layer has two subbands consisting of a light hole band LH and a heavy hole band HH which has a higher energy level than the light hole band LH. The electrons which are introduced into the light-generating layer with an electric current are stored in the conduction band while the holes introduced into the light-generating layer are stored mainly in the heavy hole band HH, so that excited particles having a relatively high energy level are generated in the light-generating layer. If the electric current applied to the light-emitting diode is increased for increasing its optical output, the number of the excited particles having the high energy level increases with an increase in the magnitude of the electric current, thereby causing the continuously induced or stimulated emission in the light resonator, which results in the laser oscillation.
It is therefore an object of the present invention to provide a quantum well type light-emitting diode having a light reflector in which there is disposed at least one light-generating layer of a quantum well structure, which light-emitting diode does not suffer from laser oscillation even when the light-emitting diode provides a relatively large optical output.
As a result of an extensive study by the inventor of the present invention, it has been found that if the light-generating layer is given a compressive strain in its direction of thickness, due to a difference of the lattice constant of the at least one light-generating layer from that of two layers which are located adjacent to and on opposite sides of the light-generating layer, the energy levels of the two subbands, i.e., the light-hole band LH and the heavy hole band HH in the valence band of the light-generating layer are made close to each other, to thereby prevent the laser oscillation. The present invention was made based on this finding.
The above-described object of the present invention may be attained according to a first aspect of the invention, which provides a quantum well type light-emitting diode having a light-emitting portion and formed of a plurality of semiconductor layers including at least one light-generating layer of a quantum well structure for generating a light, and a pair of reflecting layers between which the at least one light-generating layer is interposed for reflecting the light generated by the at least one light-generating layer, so that the pair of reflecting layers functions as a light resonator, the quantum well type light-emitting diode emitting the light generated by the light-generating layer from the light-emitting portion, wherein the improvement comprises: each of the at least one light-generating layer having a lattice constant which is smaller than that of at least two semiconductor layers of the plurality of semiconductor layers, which two semiconductor layers are located adjacent to and on opposite sides of the at least one light-generating layer.
In the light-emitting diode constructed according to the above first aspect of the present invention wherein the lattice constant of the light-generating layer is smaller than that of the two layers which are located adjacent to and on the opposite sides of the light-generating layer, the light-generating layer having a relatively small thickness is strained in a direction parallel to its plane, such that the lattice interval of the crystal of the light-generating layer is equal to that of the crystal of the adjacent two layers which are located on the opposite sides of the light-generating layer, whereby the light-generating layer is given a compressive strain acting in its thickness direction. In this state, the energy levels of the light hole band LH and the heavy hole band HH in the valence band of the light-generating layer are made close to each other, so that the holes introduced into the light-generating layer are stored or accumulated in both of the light and heavy hole bands LH and HH of the valance band, resulting in a lowered level of the excitation energy of the holes. Accordingly, the present arrangement advantageously prevents the laser oscillation even when a large amount of holes are introduced into the light-generating layer upon application of a relatively large amount of electric current to the light-emitting diode since the number of the high-energy particles which cause the continuously induced or stimulated emission is relatively small.
The above-described object of the present invention may also be attained according to a second aspect of the invention, which provides a quantum well type light-emitting diode having a light-emitting portion and formed of a plurality of semiconductor layers including at least one light-generating layer of a quantum well structure for generating a light, and a pair of reflecting layers between which the at least one light-generating layer is interposed for reflecting the light generated by the at least one light-generating layer, so that the pair of reflecting layers functions as a light resonator, the quantum well type light-emitting diode emitting the light generated by the light-generating layer from the light-emitting portion, wherein the improvement comprises: each of the at least one light-generating layer being given a compressive strain and is thereby compressed in a direction of thickness thereof.
In the thus constructed light-emitting diode wherein the light-generating layer is given a compressive strain acting in its thickness direction, the energy levels of the light hole band LH and the heavy hole band HH in the valence band of the light-generating layer are made close to each other, so that the introduced holes are stored in both of the light and heavy hole bands LH and HH of the valence band, resulting in a lowered level of the excitation energy of the holes. Accordingly, the present arrangement advantageously prevents the laser oscillation even when a large amount of holes are introduced into the light-generating layer upon application of a relatively large amount of electric current to the light-emitting diode since the number of the high-energy particles which cause the continuously induced or stimulated emission is relatively small.
The above-described object of the present invention may also be attained according to a third aspect of the present invention, which provides a quantum well type light-emitting diode having a light-emitting portion and formed of a plurality of semiconductor layers including at least one light-generating layer of a quantum well structure for generating a light, and a pair of reflecting layers between which the at least one light-generating layer is interposed for reflecting the light generated by the at least one light-generating layer, so that the pair of reflecting layers functions as a light resonator, the quantum well type light-emitting diode emitting the light generated by the light-generating layer from the light-emitting portion, wherein the improvement comprises: each of the at least one light-generating layer having a valence band which is split into two subbands consisting of a light hole band (LH) and a heavy hole band (HH), the light hole band and the heavy hole band having energy levels which are substantially equal to each other.
In the thus constructed light-emitting diode wherein the energy levels of the light hole band LH and the heavy hole band HH in the valence band of the light-generating layer are made substantially equal to each other, the holes which are introduced into each light-generating layer are stored or accumulated in both of the light and heavy hole bands LH and HH. Accordingly, the present arrangement advantageously prevents the laser oscillation even when a large amount of holes are introduced into the light-generating layer upon application of a relatively large amount of electric current to the light-emitting diode since the number of the high-energy particles which cause the continuously induced or stimulated emission is relatively small.
In one preferred form of the above first, second, and third aspects of the present invention, the light-generating layer is given the compressive strain within a range of 0.4-0.5%. According to this arrangement, the energy levels of the light and heavy hole bands LH and HH in the valence band of the light-generating layer are made substantially equal to each other. If the light-generating layer is given the compressive strain smaller than 0.4%, the energy level of the light hole band LH is not as high as that of the heavy hole band HH. If the light-generating layer is given the compressive strain larger than 0.5%, the energy level of the light hole band LH is excessively higher than that of the heavy hole band HH, undesirably causing the laser oscillation.
In another preferred form of the above-described first, second, and third aspects of the present invention, the pair of reflecting layers consists of a first reflecting layer and a second reflecting layer, the first reflecting layer consisting of a multiplicity of unit semiconductors which are formed by crystal growth on a monocrystalline GaAs substrate and which constitute a first distributed-Bragg reflector which is located on the side of the substrate, each of the at least one light-generating layer of a quantum well structure being interposed between two semiconductor barrier layers which are formed by crystal growth, the second reflecting layer consisting of a multiplicity of unit semiconductors which are formed by crystal growth on an upper one of the two barrier layers between which an uppermost one of the at least one light-generating layer is interposed, the multiplicity of unit semiconductors of the second reflecting layer constituting a second distributed-Bragg reflector which is located on the side of the light-emitting portion.
According to this arrangement, the light generated in the light-generating layer is emitted from the light-emitting portion of the light-emitting diode after the light is resonated in the light resonator constituted by the pair of the reflecting layers between which the light-generating layer is interposed.
In still another preferred form of the above-described first through third aspects of the present invention, the at least one light-generating layer is formed in the light-emitting diode such that each of the at least one light-generating layer is aligned with a corresponding one of at least one antinode of a standing wave generated in the light resonator.
This arrangement effectively increases the quantum efficiency of the light-emitting diode, resulting in a significantly high degree of light emission output of the diode.